1. Field of the Invention
The present invention relates to a technique for feeding sheets of paper, contained stacked in a paper feed cassette, one by one from the top layer to a paper feed path and, more specifically, to a technique improving possibility of recovery to a normal paper feeding state.
2. Description of the Background Art
As is well known, an image forming apparatus such as a copy machine is provided with a paper feeder for feeding sheets of paper contained stacked in a paper feed cassette one by one from the top layer to the paper feed path.
The paper feeder includes a paper feed unit for feeding the sheets one by one from the paper feed cassette to the paper feed path, and a paper feed sensor for detecting whether or not the sheet has actually been fed out from the paper feed unit. Typically, the paper feed unit is provided with a pick-up roller that is brought into contact with the uppermost one of the sheets stored in the paper feed cassette, and retard rollers implemented by a paper feed roller and a separation roller positioned one above the other to form a pair.
In the paper feeder, when a start key of the image forming apparatus is operated, the pick-up roller, and the paper feed roller and separation roller start to rotate in a direction of feeding the sheet of paper. One sheet, which is separated and taken out as it passes through the retard rollers, passes through the paper feed sensor, and thereafter it is transmitted by a sheet feeding unit to an image forming unit.
If the paper feed sensor does not detect presence of any sheet even after a prescribed time period from the start of sheet feeding, it is determined that paper feed failed because of a paper jam at the paper feed unit, an indication is given accordingly on a display panel of an operation unit, and the operation of the apparatus is stopped. For recovery from this state, an operator opens a cover provided on a front side of the copy machine, removes the jammed paper, aligns the sheets in the paper cassette, and presses a reset switch again.
In the paper feeder having such a structure, whenever the paper feed sensor fails to detect any sheet within a prescribed time period, it is determined that paper jam occurred and the machine operation is stopped, and the recovery work described above is required each time.
Paper feed failure, however, may occur not because of the jam but because of life of the paper feed roller. If the paper feed fails because of the life of paper feed roller, it is often the case that normal paper feed may be resumed after repeating a few paper feed operations.
A paper feeder as a solution to the problem is disclosed in U.S. Pat. No. 5,395,106. In the paper feeder, if a sheet fed from the paper feed cassette is not detected by the paper feed sensor, the paper feeding operation of the paper feed unit is repeated for a prescribed number of times. According to this technique, unnecessary suspension of the feeder operation can be avoided at the time of paper feed failure.
The paper feeder, however, has the following disadvantages.
Referring to FIG. 1, assume that a pick-up roller 2000, and a paper feed roller 2002 and a separation roller 2004 forming a vertical pair are rotated in a paper feeding direction to realize the paper feeding operation. Here, it is possible that a sheet 2006 on a lower side is fed to rollers 2002 and 2004 serving as retard rollers, overlapped with an upper sheet 2008 with the tip end of lower sheet 2006 protruded to the downstream side of the paper feeding direction than the tip end of upper sheet 2008 (that is, lower sheet 2006 goes ahead of upper sheet 2008). This situation is shown in FIG. 1A.
In this state, when rotation of roller 2004 is reversed to the sheet returning direction to separate the sheets, the lower sheet 2006 is fed by roller 2002 to the downstream side of paper feeding direction and its tip end comes to protrude from a nip formed by rollers 2002 and 2004. Consequently, paper feed sensor 2010 is once turned ON. Thereafter, the lower sheet 2006 that has turned ON the sensor 2010 is returned to the upstream side of paper feeding direction by roller 2004, and thus, sensor 2010 is turned OFF. At this time, the two sheets fed overlapped with each other come to be kept as they are at the nip formed by rollers 2002 and 2004, with the tip end of upper sheet 2008 slightly protruding to the downstream side of paper feeding direction than the tip end of lower sheet 2006. As a result, feed sensor 2016 arranged downstream side of the conveyor rollers 2012 and 2014 in the paper feeding direction is kept OFF. Specifically, the state of FIG. 1B changes to that of FIG. 1C.
As described above, dependent on the state of overlapped feeding of sheets, it is possible that presence of a sheet fed from the paper feed cassette by the paper feed operation is once detected by a sensor 2010 provided on the paper feed path and after a sheet separating operation, the presence of the sheet may not be detected by the sensor 2010 any longer. Though the normal state may be recovered by another paper feed operation, such a situation is determined to be a paper jam before reaching feed sensor 2016, an indication to that effect is given on the display panel of the operating unit, and the operation of the apparatus is stopped.